Method and device for in situ triaxial test

ABSTRACT

The device comprises a corer ( 1, 4 ) fitted with a flexible diaphragm ( 2 ) on its inside face, means ( 10 ) for driving the corer into the soil ( 11 ), means for breaking up the soil leaving the top orifice of the corer, means for applying axial forces on the diaphragm ( 2 ) and on the top face of the soil sample ( 12 ) contained in the corer, and means for measuring stresses and deformations to which the sample ( 12 ) is subject. While a measurement is being performed, the corer is held stationary in the soil ( 11 ).

PRIORITY CLAIM

This is a U.S. national stage of application No. PCT/FR00/03050, filedon Nov. 2, 2000. Priority is claimed on that application and on thefollowing application(s): Country: France, Application No.: 99 13792,Filed: Nov. 4, 1999.

The invention relates to a method for measuring the deformationmoduluses of a soil sample in situ under conditions that are close tothose of a triaxial compression test, and it also relates to apparatusfor implementing the method.

Traditionally, direct measurement of the deformation moduluses of soilis performed by a “circularly-symmetrical triaxial compression test”method. For this purpose, a corer is used to take soil cores, and theuse of a corer is not without incidence on the quality of the sampleobtained. In spite of the sophistication of corers, it is found thatreorganization occurs when the core penetrates into the casing becauseof the arching effect created by friction between the soil and theinside wall of the casing. The intermediate stages between taking thecore and performing the test proper allow the initial stresses in thesample to relax. In order to test a sample that is remote from thissuspected reorganized boundary region, the sample is generally cut downwell away from the outside edges of the core. This precaution gives riseto a significant loss of material.

Laboratory testing, referred to as triaxial compression testing, is ahomogeneous test that serves to identify the deformation moduluses ofthe soil that are required for deformation calculations, e.g. of the“finite element” type.

Triaxial compression testing consists in placing the test sample (whichis generally cylindrical in shape having a section that is circular orsquare) in a cylindrical bag that is expandable. The bag is placed in anenclosure full of liquid whose pressure can be adjusted, and it isplaced between two pistons which can exert forces on the end faces ofthe sample. Measurement devices serve to measure the displacements ofthe wall of the bag, the displacements of the pistons, and the pressuresinside the enclosure and inside the soil. The deformation modulus ismeasured either by increasing the pressure inside the enclosure and theforces applied to the piston, in which case it is variation in thevolume of the sample that is measured, or else by injecting a knownvolume of liquid into the enclosure and then measuring the resultingvariation of pressure inside the sample.

U.S. Pat. Nos. 4,502,338 and 4,579,003 describe instruments for testingthe samples under triaxial compression conditions.

Triaxial compression testing serves to measure the deformation modulusesdirectly. It also serves to monitor drainage conditions and to determinethe anisotropy parameters of the soil under test.

Nevertheless it suffers from various defects. Firstly it is cumbersometo implement and consequently expensive since the idea is to study soilwhile it is still intact.

It also requires a great deal of time since it is necessary during theconsolidation stage to recreate the initial stress field prior toperforming the test. The result is also liable to be biased by variouserrors, due to slack in the contact surfaces and to errors of axialalignment in the mechanical stack constituted by the test machine.

In order to avoid some of those drawbacks, proposals have already beenmade to perform soil strength testing in situ, by means of tools thatare expanded in a borehole.

FR 1 596 747 thus proposes a pressure-measuring boring sonde whichcomprises a sonde body provided on its outer perimeter with a flexiblediaphragm and in which there is placed a hollow rod, and a cutting toolconstituted by a sonde body and by a soil breakup member disposed in thehollow body and secured to the hollow rod.

That self-boring sonde can perform measurements as the boreholeadvances. However, the drawback of the measurements performed with thatsonde is that the pressure-measuring test is based on the sondeexpanding in a cylindrical cavity formed in soil of infinite dimensions.Unfortunately, under such conditions, the stress state generated in thesoil is not homogeneous, and as a result deformation moduluses can beobtained only by applying empirical relationships which are difficult todevelop.

Use of the pressure-measuring test has become a requirement whendimensioning foundations. Nevertheless, in the field of retaining earthand landslides, laboratory testing still remains essential.

The object of the invention is to provide a method and apparatus thatmake it possible, in situ, to establish a stress field that is uniformin a finite volume of soil under test, while avoiding any movement ofthe sample of soil for testing before the actual beginning of each test.

Another object is to propose a method and apparatus for in situmeasurement of the moduluses of deformation of a soil sample undercircularly-symmetrical triaxial compression testing conditions which canbe used in particular in the field of retaining earth and landslides.

According to the invention, the method is characterized by the followingsteps:

driving a corer into the soil, the corer being fitted on its inside facewith a flexible diaphragm;

removing the material that leaves the corer through its top orifice asit is being driven into the soil, so as to form a sample of soil fortesting inside the corer;

holding the corer stationary in the soil in order to proceed with ameasurement;

exerting pressure forces on the outside face of the diaphragm and on thetop face of the sample so as to subject the sample to radial and axialstresses; and

measuring the deformations to which the sample is subject.

Preferably, during measurement, the sample is subjected to predeterminedradial stresses, and the axial stresses are varied until the samplebreaks.

In order to determine the anisotropy parameters of the soil, a corer isused having an inside section that is substantially square.

The apparatus of the invention for implementing the method comprises:

a corer fitted on its inside face with a flexible diaphragm;

means for driving said corer into the soil in order to form a sampleinside said corer;

means for removing the material which leaves said corer through its toporifice while the corer is being driven into the soil;

means for holding said corer stationary while taking a measurement;

means for applying an axial force to the top face of the sample;

means for applying pressure to the outside face of the flexiblediaphragm;

means for measuring the radial and axial stresses to which the sample issubjected; and

means for measuring the deformations to which the sample is subject.

Other advantages and characteristics of the invention appear on readingthe following description made by way of example and with reference tothe accompanying drawings, in which:

FIG. 1 is a section view on a vertical axial plane through apparatus ofthe invention while performing a measurement in situ;

FIG. 2 is a diagrammatic section view through the FIG. 1 apparatus whileit is being driven into the soil, showing the means for breaking up thesoil on leaving the corer; and

FIG. 3 is a diagrammatic section view through a variant embodiment ofthe apparatus.

The hollow body 1 of the sonde has a narrowing in its inside faceserving as a housing for a flexible and expandable diaphragm 2 which, inthe absence of any pressure forces on its outside face, lies in linewith the inside wall 3 of a cutting shoe 4 fixed on the bottom end ofthe hollow body 1 by means of a screw thread 6. The cutting shoe 4 canthus be replaced in the event of its bottom end 7 of chamfered sectionbecoming worn.

The top end of the hollow body 1 is connected to a tool head 8 whichincludes fixing means 9 on the bottom end of a jack rod 10 actuated bysuitable means disposed on the surface of the soil under test.

When a vertical force F is exerted on the rod 10, the cutting shoe 4penetrates into the soil 11 like a corer, and a sample 12 of soilpenetrates into the cavity of the cutting shoe 4 and of the hollow body1.

A passage 13 is formed in the inside wall of the hollow body 1, whichpassage opens out slightly above the top plane P1 of the diaphragm 2 viaan orifice 14 having a jetting system mounted therein, which system isfed under high pressure by a slip of bentonite, for example, thusserving to break up the core of soil leaving the measuring zone. Thepassage 13 is connected to the surface via a duct 15 which delivers theslip.

The chamber 16 that exists between the diaphragm 2 and the inside faceof the hollow body can be fed with fluid under pressure delivered fromthe surface via a duct 17 and a passage 18 formed in the hollow body 1.The chamber 16 also includes detector means 19 for detectingdisplacement of the diaphragm 2, and pressure measuring means, whichmeans are connected to the surface by cables.

The tool head 8 includes a cylindrical cavity 20 about a vertical axishaving a double-acting drive piston 21 mounted therein with the pistonrod 22 passing through the bottom face of the tool head 8. The bottomend of the piston rod is fitted with a second piston 23 whose diameteris substantially equal to the diameter of the hollow body 1. Verticaldisplacement of the drive piston 21 drives vertical displacement of thesecond piston 23. This second piston can take up a high position asshown in FIGS. 2 and 3 where it is situated above the plane of theorifice 14, which position is used while the sonde is being driven intothe ground, and a low position as shown in FIG. 1 in which the bottomface is substantially level with the plane P1 and can exert a force onthe top face of the sample 12 held captive in the diaphragm 2 and thecutting shoe 4. This low position is used during a measurement, and theforce is generated by the drive piston 21. The two chambers of thecylindrical cavity 20, as separated by the drive piston 23 are connectedto a hydraulic circuit which is controlled from the surface and which isnot shown in the drawings.

The tool head 8 is connected to the hollow body 1 by arms 25 which leavebetween them passages 26 through which the mixture of slip and soil asbroken up by the jetting system rises to the surface.

A measurement is taken as follows:

While the sonde is being driven into the soil, the chamber 16 isunpressurized, the second piston 23 is in its high position, and thejetting system is set into operation. The sonde is driven into theground by the jack rod 10 through a depth that is not less than theheight of the sonde. The sample 12 of the preceding measurement isbroken up by the jetting system and a new sample 12 forms inside thediaphragm 2.

At the end of this driving operation, the jack rod 10 is heldstationary, the jetting system is stopped, and the second piston 23 ispressed against the top face of the sample 12 with a small amount offorce.

Thereafter the measurement proper is performed. To do this, the chamber16 is put to a predetermined pressure P. The sample 12 is thus subjectedto predetermined radial stress. Thereafter the axial stress applied tothe sample 12 is varied by varying the force exerted on the top face ofthe sample 12 by means of the drive piston 21. During testing, theradial stress is kept constant and it is the axial stress whichincreases until the sample 12 breaks in shear.

By its very design, the proposed apparatus makes it possible to imposestress paths. This is done by servo-controlling the second piston 23 andthe lateral pressure chamber 16. By making local measurements, it ispossible to reach the range of small deformations. Local measurementsare obtained by placing the sensors as close as possible to the sample12, i.e. sensors of radial and axial displacements, together with aninterstitial pressure sensor, all of which are fixed to the diaphragm 2.

In the embodiment described above, the hollow body 1, the cutting shoe4, and the second piston 23 are circular in section.

The hollow body 1, the cutting shoe 4, and the second piston 23 could beof square section. Such apparatus makes it possible to implement agenuine triaxial compression test, and to determine the parameters ofthe anisotropy of the sample 12 under test.

FIG. 3 shows a variant embodiment of the cylindrical type in which thesonde body 1 has a breaking-up tool 30 on the outside and set back fromthe cutting shoe 4, which tool forms a well 31. This variant embodimentmakes it possible to test materials of a variety of kinds that aredifficult to take as samples, and also to test materials that are quitecoarse.

The chamfered edge of the cutting shoe 4 is such that the inside face 3of the bottom end of the shoe 4 is cylindrical, while the outside face32 is conical and serves to push untested soil outwards away from thesonde while the sonde is being driven into the soil. The inside face 3serves to smooth the sides of the sample 12 which have negligibleinfluence on the measurements taken.

The apparatus of the invention can be used to perform a test that issimilar to the circularly symmetrical triaxial compression test asperformed in a laboratory. However efficiency is considerably improvedbecause testing is performed step by step and almost continuouslywithout large amounts of maneuvering.

What is claimed is:
 1. A method of measuring in situ the deformation moduluses of a soil sample (12) under conditions close to those of a triaxial compression test, the method being characterized by the following steps: driving a corer (1, 4) into the soil (11), the corer being fitted on its inside face with a flexible diaphragm (2); removing the material that leaves the corer (1, 4) through its top orifice as it is being driven into the soil, so as to form a sample (12) of soil for testing inside the corer; holding the corer (1, 4) stationary in the soil in order to proceed with a measurement; exerting pressure forces on the outside face of the diaphragm (2) and on the top face of the sample (12) so as to subject the sample to radial and axial stresses; and measuring the deformations to which the sample (12) is subject.
 2. A method according to claim 1, characterized by the fact that during measurement, the sample (12) is subjected to predetermined radial stresses, and the axial stresses are varied until the sample (12) breaks.
 3. A method according to claim 1, characterized by the fact that a corer (1, 4) is used having an inside section that is substantially square.
 4. Apparatus for measuring in situ the deformation moduluses of soil under the conditions of a triaxial compression test, the apparatus being characterized by the fact that it comprises: a corer (1, 4) fitted on its inside face with a flexible diaphragm (2); means (10) for driving said corer into the soil (11) in order to form a sample (12) inside said corer; means for removing the material which leaves said corer through its top orifice while the corer is being driven into the soil (11); means for holding said corer stationary while taking a measurement; means for applying an axial force to the top face of the sample (12); means for applying pressure to the outside face of the flexible diaphragm (2); means for measuring the radial and axial stresses to which the sample (12) is subjected; and means for measuring the deformations to which the sample (12) is subject.
 5. A method according to claim 2, characterized by the fact that a corer (1, 4) is used having an inside section that is substantially square. 